Most onboard USB hubs have a dedicated crystal oscillator but on some
boards the clock signal for the hub is provided by the SoC.

In order to support this, we add the possibility of specifying a
clock in the devicetree that gets enabled/disabled when the hub
is powered up/down.
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Link: https://lore.kernel.org/r/20231127112234.109073-2-frieder@fris.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>